As we can see basically we are opening external ports on the router, and allowing that port to be routed back to our internal network on the specified IP address. (Remember to substitute your internal IP address above and the port you wish to use. You may also need to change the Dialer, either Dialer1 or Dialer0 depending how your device is configured)

Re-Direct Forwarding

In your internal network you may have 3 machines which all run RDP on port 3389. Now you cannot map 3389 three times, only 1 port can be used at a time.

So what you can do, is setup a redirection so that the external port numbers are different but map to the same 3389 port numbers internally.